In a typical radio communications network or cellular network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. A cell may be downlink (DL) and/or uplink (UL) cell. DL means communication from the base station to the UE and UL means communication from the UE to the base station. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
Device-to-Device (D2D) communication is widely used as a component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite such as WiFi Direct. These systems operate in unlicensed spectrums of radio resources.
Recently, D2D communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating communication devices and at the same time to allow communication devices to operate in a controlled interference environment. Typically, it is suggested that such D2D communication shares the same spectrum of radio resources as the cellular network, for example by reserving some of the cellular uplink radio resources for D2D purposes. Allocating dedicated spectrum of radio resources for D2D purposes is a less likely alternative as spectrum of radio resources is a scarce radio resource and, dynamic, sharing between the D2D services and cellular services is more flexible and provides higher spectrum efficiency.
Communication devices that want to communicate, or even just discover each other, typically need to transmit various forms of direct signals/channels. The radio resources for such channels/signals may be assigned by a third controlling node such as an eNB or a control communication device, or the radio resources could be selected autonomously by the transmitting communication device, possibly within a restricted pool of available radio resources.
Multiple direct signals/channels from different communication devices are multiplexed on the same radio resources in a combination of Time Division Multiplexing (TDM), Frequency Division Multiplexing (FDM) and possibly Code Division Multiplexing (CDM). Even though details are not agreed yet, it is likely that D2D signals/channels will be multiplexed on specific subframes occurring at known, possibly signaled or configured, positions in the radio frame. This is mainly to limit the active time for the receiver.
Radio resources for transmission of data, and control channels, including discovery, may be assigned by a controlling node or be defined according to pre-configured patterns. In general, each channel from each communication device occupies a subset of the time/frequency and possibly code radio resources in the system.
Channels with different purpose or associated to, e.g., different service types may be associated to different bandwidths and/or number of Physical Radio resource Blocks (PRB).
It is also important to note that UL and D2D transmissions in LTE are assumed to exploit Single Carrier Orthogonal Frequency Division Multiplexing (SC-OFDM), i.e., single carrier modulation. This means that a given communication device can only transmit over a single contiguous subset of spectral radio resources at a given time.
It is also observed that, especially when D2D communication is performed under network coverage, UL and D2D transmissions may be possibly co-scheduled in FDM in the same subframe.
Frequency Hopping
Since interference at the receiver in a D2D system happens in a stochastic and partially uncontrollable/unpredictable fashion, it is understood [2] that frequency and/or time diversity is beneficial in the radio resource patterns used for each physical channel. In particular, the channels should span different portions of the spectrum in a pseudo-random fashion, a so called frequency hopping. Possibly, code patterns may be exploited, too.
For certain channels, feedback-based acknowledgement mechanism may not be available, e.g., control channels, broadcast communication channels, discovery channels, etc. In these cases, a possible approach is to provide blind retransmissions, i.e., transmit the same payload multiple times on different radio resources, possibly with different encoding parameters, e.g. redundancy versions. Under certain conditions the receiver might be able to reconstruct the correct information based on reception of at least some of the retransmissions of the same packet. In order to maximize the detection probability, it is useful to provide frequency hopping between packet retransmissions.
Frequency Hopping (FH) is known since LTE Rel-8, where hopping patterns that sample the frequency spectrum in a pseudo-random fashion are defined and assigned to transmitting communication devices.
The combination of radio resource patterns with frequency hopping and possibly channels with different bandwidth requirements results in an inefficient use of the spectrum of radio resources.